1) Field of the Invention
The present invention relates to an optical receptacle, optical sub assembly and optical transceiver suitable for use in TOSA (Transmitter Optical Sub Assembly) or ROSA (Receiver Optical Sub Assembly) to be mounted in a pluggable type optical transceiver module such as SFP (Small Form-Factor Pluggable) or XFP (10 Gigabit Small Form-Factor Pluggable).
2) Description of the Related Art
In recent years, along with the establishment of the MSA (Multi Source Agreement) for a pluggable type optical transceiver module such as SFP or XFP, it has come into widespread use in optical communication systems based upon SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) and Ethernet (registered trademark). In this pluggable type optical transceiver module, optical assemblies with transmission and reception functions, such as TOSA (Transmitter Optical Sub Assembly) and ROSA (Receiver Optical Sub Assembly), are mounted in packages specified in dimension according to the MSA.
For example, as shown in FIG. 14, a TOSA 100 for the SFP is made up of a semiconductor laser element (hereinafter referred to as an LD element) 101, lenses 102 and 103 for forming a laser beam, emitted from the LD element 101 into a parallel light and a focused light, a package 104 for mounting parts including other electronic parts and a component comprising an optical receptacle 105 for making the connection and introduction of the laser beam to and into a optical connector plug (in the case of the SFP, an LC connector plug, see reference numeral 130 in FIG. 16).
The optical receptacle 105 includes a fiber stub 108, a holder 109 made of a metal or the like, a tubular sleeve 110 and a sleeve case 111 made of a metal or the like. The fiber stub 108 is constructed in a manner such that an optical fiber 107 mainly made of a quartz glass is fixedly inserted into a through hole made in a cylindrical ferrule 106 made of a ceramic material such as zirconia.
In addition, the optical receptacle 105 has a structure in which a rear end side of the fiber stub 108 is inserted into the holder 109, made of a metal or the like, under pressure to be fixed therein and a portion of a cylindrical part forming the sleeve 110 is inserted into a tip side to be embedded therein. Moreover, for preventing the sleeve 110 from protruding, the sleeve case 111 is provided outside the sleeve 110, and this sleeve case 111 is also inserted into the holder 109 to be integrated therewith.
Furthermore, for example, as shown in FIG. 15, an ROSA 120 for the XFP includes an optical receptacle 112 for making a connection of an optical connector plug (in the case of the XFP, an LC connector plug; see reference numeral 130 in FIG. 16), a semiconductor light-receiving element (hereinafter referred to as a PD element) 113, a lens 114 for focusing a laser beam from the optical connector plug on the PD (Photo Diode) element 113 and a package 115 for mounting parts including other electronic parts.
The optical receptacle 112 shown in FIG. 15 has an example of a structure different from that of the optical receptacle 105 shown in FIG. 14, and it includes a tubular sleeve 117, a transparent glass plate 116 fixedly secured through adhesion or the like to an inner surface of the sleeve 117, a sleeve case 119, and a holder 118 for making the precise fixing for integration between the sleeve 117 and the sleeve case 119.
On the other hand, as shown in FIG. 16, an optical connector plug (terminal member) 130 is inserted and fitted in the optical receptacle 105 or 112 of the TOSA 100 or the ROSA 120, and it includes a plug ferrule (plug body) 122 where an optical fiber 121 is inserted into a through hole made in its own central portion, a spring 123, which is an elastic body, for making a close adhesion between the plug ferrule 122 and the fiber stub 108 or the glass plate 116 in a manner such that the plug ferrule 122 is physically pressed against the fiber stub 108 or the glass plate 116 by a predetermined force, and a plug housing 124 for accommodating these parts internally.
In recent years, along with the widespread use of the pluggable type optical transceiver module such as SFP or XFP in the field of data communications such as Ethernet, there is a requirement for no occurrence of problems on performance even when it is handled as well as conventional electric connectors. In particular, an increase in the number of optical lines or the number of optical fiber cords encounters an increase in the weight of a plurality of optical fiber cords bundled, so there is a need for almost no variation of the characteristic even if a load is imposed on the optical fiber cord which is in a connected condition. Concretely, for example, there is a need to satisfy a specification in which, for example, the optical output fluctuation in the TOSA (or the reception sensitivity in the ROSA) is within a predetermined variation range (below 1 dB) when a predetermined load (for example, 100 gf) is applied to an optical fiber cord.
For reducing this characteristic variation at the application of a load to the cord, in addition to the structure of a housing of the pluggable type optical transceiver module, it is considered that the structure of an optical receptacle of an optical sub assembly such as the TOSA, particularly a sleeve (see reference numerals 110 and 117 in FIGS. 14 and 15) in the optical receptacle, fulfills an extremely important role. The sleeves commonly put into practical use are roughly classified into a slit sleeve 141 shown in FIG. 17 and a precision sleeve (solid sleeve) 142 shown in FIG. 18.
The slit sleeve 141 shown in FIG. 17 is, in the literature, of a type that a split (slit) 141a is made in a tubular sleeve 141, and the inner diameter R1 of the slit sleeve 141 is set to be slightly smaller than the outer diameters of a fiber stub (see reference numeral 108 in FIG. 14) and a plug ferrule (see reference numeral 122 in FIG. 16). Therefore, the fiber stub and the plug ferrule 122 (see FIG. 16) can be inserted into the slit sleeve 141 by a predetermined force and, after the insertion thereinto, the plug ferrule can be held by its elastic force (closing force against an opening force on the split 141a).
Taking note of the optical receptacle 105 shown in FIG. 14, since the fiber stub 108 is inserted into the holder 109 under pressure and fixed therein, when the slit sleeve 141 shown in FIG. 17 is fitted in both the fiber stub 108 and the inserted plug ferrule 122, the plug ferrule 122 is aligned (lined up) along an outer circumference of the fiber stub 108 by the elastic force of the slit sleeve 141, which enables the axial alignment between optical fibers 107 and 121 which lie at the central portions of both the fiber stub 108 and the plug ferrule 122.
It is not necessarily preferable that he slit sleeve 141 has a higher elastic force. That is, an excessive elastic force enhances a force (usually, a force needed for drawing out the plug ferrule 122, and referred to withdrawal power) needed for the insertion and extraction of the plug ferrule 122. The enhancement of the withdrawal power makes it difficult for the plug ferrule 122 to abut on the fiber stub 108 by the force of the spring 123 as mentioned above, so the plug ferrule 122 and the fiber stub 108 does not reach the close adhesion, which can cause the occurrence of reflection and coupling loss. For this reason, according to the rules such as IEC (International Electrotechnical Commission) and FOCIS (Fibber Optic Connector Intermateability Standards), for example, in the case of an LC connector, the withdrawal power is set to be 1 to 2.5N in order to avoid the excessive power.
On the other hand, the precision sleeve 142 has no slit as shown in FIG. 18, and the inner diameter R2 thereof is precisely processed to be slightly larger than the outer diameters of the fiber stub 108 and the plug ferrule 122. Accordingly, the axial alignment between optical fibers in the fiber stub 108 and the plug ferrule 122 can be made precisely with an accuracy of approximately 1 μm.
A description will be given hereinbelow of a behavior in a case in which, assuming that an optical connector plug 130 is connected to the TOSA 100 shown in FIG. 14, a load is applied to an optical fiber cord coupled to the optical connector plug 130. The optical connector plug 130 somewhat moves backwards when the plug ferrule 122 abuts on the fiber stub 108 at the insertion (fitting) into the TOSA 100 and, in this state, in most cases, the optical connector plug 130 does not come into contact with any portion within the plug housing 124, that is, it is in a so-called floating state.
However, a large load is applied to a fiber cord, the floating state breaks so that a portion of the load can be applied directly to the plug ferrule 122 as shown in FIG. 19. In such a state, when the slit sleeve 141 shown in FIG. 17 is used as the sleeve 110 for the optical receptacle 105, since the elastic force of the slit sleeve 141 becomes smaller than the load of the plug ferrule 122, the split 141a of the sleeve 141 can be opened so that the axis of the optical fiber 121 within the plug ferrule 122 shifts from the axis of the optical fiber 107 included in the fiber stub 108.
In such a situation, a coupling loss occurs, and in the case of the TOSA, the optical output fluctuates. Thus, in the case of the slit sleeve 141, due to its structure, the slit sleeve 141 can fall into an excessively opened state (its diameter increases) when a load exceeding a predetermined value is applied to the plug ferrule 122, thereby enhancing the characteristic variation at the application of a load to the cord.
On the other hand, the precision sleeve 142 has no slit as mentioned above and, even if a load exceeding a predetermined value is put on the plug ferrule 122, the sleeve 142 does not fall into an opened state (no enlargement of the diameter) except that the precision sleeve 142 breaks down, so the characteristic variation at the application of a load to a cord in the case of the precision sleeve 142 becomes better in comparison with the slit sleeve 141. In particular, in a case in which there is a need to reduce the characteristic variation at the application of a load to the cord, the precision sleeve 142 is sometimes put to use.
As other conventional techniques related to the invention of the subject application, there are techniques disclosed in the following Patent Documents 1 and 2.
The Patent Document 1 discloses a technique on a sleeve structure having an elastic section where a slit is made in a longitudinal direction and a rigid section with no slit which is caulked with a gripping ring.
In addition, the Patent Document 2 discloses a structure of a precision sleeve in which a slit (split) having a width at its central portion wider than a width at its open-end portion is formed from the open-end portion to the central portion in a longitudinal direction.
Patent Document 1: Japanese Patent Laid-Open No. 2004-317848
Patent Document 2: Japanese Patent Laid-Open No. 2005-181903
However, the inventor of the subject application found that the precision sleeve 142 has the following problems.
In the case of the precision sleeve 142, since its inner diameter is larger by only several μm than the outer diameters of a fiber stub and a plug ferrule as mentioned above, the coupling therebetween is not easy. For this reason, in a case in which, for example, the precision sleeve 142 is applied to the sleeve 110 of the optical receptacle 105 as shown in FIG. 19, if the optical connector plug 130 is obliquely inserted into the optical receptacle 105, the tip portion of the plug ferrule 122 gnaws on an entrance portion of the precision sleeve 142 to be caught thereon (see A1 in FIG. 19).
If the insertion of the optical connector plug 130 is further made in a state where the tip portion of the plug ferrule 122 is caught on the entrance portion of the sleeve 110 serving as the precision sleeve, the spring 123 within the optical connector plug 130 is contracted so as to push the plug ferrule 122 into the interior. Thus, when the insertion angel of the optical connector plug 130 is changed as shown in FIG. 20 in the state of the contraction of the spring 123 in the optical connector plug 130, the catching of the plug ferrule 122 is removed (see A2 in FIG. 20), and the plug ferrule 122 is inserted into the sleeve 110 serving as the precision sleeve.
In this case, since the plug ferrule 122 is powerfully inserted thereinto by the force of the contracted spring 123, as shown in FIG. 21, the plug ferrule 122 can violently come into collision with a surface of the fiber stub 108 (see A3 in FIG. 21). At this time, there is a problem in that a large impact exceeding, for example, 10,000 G occurs since the ferrules 106 and 122 made of a hard material such as zirconia come into collision with each other.
Such a large impact leads to that the fiber stub 108 inserted under pressure settles down or the performance of precise electronic parts mounted in the interior of the package 104 suffers degradation. Even in the case of the optical receptacle 112 mentioned above with reference to FIG. 15, almost same problem arises, and the plug ferrule 122 violently comes into collision with a surface of the glass plate 116 so as to produce a large impact exceeding 10,000 G. Such a large impact affects the performance of precise electronic parts mounted in the interior of the package 115. Incidentally, in the case of the optical receptacle 112 shown in FIG. 15, the glass plate 116 and the sleeve 117 are fixed to each other through an adhesive, and the glass plate 116 does not settle down.
In this case, hypothetically, if it is possible to remove the spring 123 on the optical connector plug 130 side, the above-mentioned problems are solvable. However, as mentioned above, the spring 123 is essential for physically bringing the plug ferrule 122 into contact with the fiber stub 108 or the glass plate 116 by a predetermined force to establish the close adhesion therebetween. No employment of the spring 123 makes it difficult to bring the plug ferrule 122 into contact with the fiber stub 108 or the glass plate 116, which leads to the occurrences of reflection and coupling loss. Therefore, the spring 123 itself is an essential member and is necessary.
As shown in FIG. 22, the aforesaid Patent Documents 1 and 2 disclose a structure in which, in the optical receptacle 150 formed by fitting the fiber stub 151 in the precision sleeve 152, a slit 153 which is a partial split is made to range from the optical connector plug insertion side of the precision sleeve 152 up to the fiber stub 151 fitted tip position.
However, in this structure, in a state of the insertion of the optical connector plug, the fiber stub is supported in the slit 153 made area of the precision sleeve 152. Accordingly, when a load exceeding a predetermined value is applied to a plug ferrule constituting the optical connector plug, as well as the above-mentioned case of the slit sleeve 141, the slit falls into an opened state, and the disposition of the plug ferrule in the insertion state of the plug ferrule becomes unstable, so the shifting of the optical axis thereof relative to the fiber stub 151 easily occurs.
That is, there is also a problem which arises with the techniques disclosed in the aforesaid Patent Documents 1 and 2 in that the characteristic variation increases at the application of a load to a cord. In fact, in a case in which a TOSA employing the optical receptacle 150 with the above-mentioned structure shown in FIG. 22 was mounted in an SFP and an LC optical fiber cord was connected thereto, when a predetermined load (for example, 100 gf) was applied to the cord and the optical output fluctuation of the TOSA was confirmed, a large fluctuation occurred, so it was of no practical use.
Moreover, for stabilizing the support of the plug ferrule in the inserted state, it is considerable that, as well as a precision sleeve 154 shown in FIG. 23, a slit 155 which is a split is formed up to an intermediate position in a longitudinal direction. However, since this structure has a limit in the enlargement of the opening portion, difficulty is encountered in sufficiently eliminating the above-mentioned catching at the insertion of the optical connector plug. Still moreover, in a case in which the slit is formed up to an intermediate position in the longitudinal direction, the repeated insertion/extraction of the plug ferrule constituting the optical connector plug causes the stress to concentrate on the cut portion forming this slit, so it is considered that cracks starting at this cut portion can occur, which creates a problem in that the lifetime of the optical receptacle becomes shorter.